This invention relates to a method of filling contact holes with aluminium for an interlayer isolation layer of a semiconductor device and additionally to apparatus for implementing the method.
A semiconductor device comprises a semiconductor substrate and an interlayer isolation or insulation layer laid on the substrate for wirings in the semiconductor device. The isolation layer has an exposed principal surface and sidewalls extended from the principal surface into the isolation layer to define the contact holes. In order to provide the wirings, a conductor film is spread on the principal surface to fill the contact holes. In general, the conductor film is made of an aluminium material consisting essentially of aluminium. The aluminium material may be a known aluminium alloy including small amounts of silicon and copper.
Various methods of spreading such a conductor or aluminium film on the principal surface to fill the contact holes are already in practical use. Examples are a bias sputter method, a high temperature sputter method, and a reflow after sputter method.
The bias sputter method comprises the steps of depositing by sputtering the aluminium material film on the principal surface in a processing chamber and of simultaneously supplying a high frequency voltage to the substrate to ionize argon gas filled in the processing chamber into argon ions. Generated argon ions etch the deposited aluminium, and the aluminium film reflows into the contact holes to fill with aluminium masses. Inasmuch as the aluminium material is sputter deposited simultanesouly with sputter etch of the aluminium film, the argon ions are taken into the conductor film as impurities. As a consequence, the conductor film has a degraded film nature, a deteriorated electromigration resistance, a morphologically objectionable planar surface, an undesirable workability, and a poor alignment.
The high temperature sputter method comprises the steps of depositing by sputtering the aluminium material film and of simultaneously heating the substrate by a heater to a high temperature for reflow the aluminium material film to provide the conductor film. In accordance with the high temperature sputter method, it is possible to keep the argon ions out of the conductor film and to avoid consequent degradation of the film nature. The aluminium material is, however, sputter deposited at the high temperature. As a result, oxygen and water vapour are taken from an atmosphere of sputter deposition into the covering film and consequently into the conductor film to give the morphologically objectionable planar surface to the conductor film. Furthermore, temperature control is difficult during the sputter deposition and is hardly reproducible.
It is belivered by the present inventor that the reflow after sputter method is far more advantageous than the bias sputter and the high temperature sputter methods. An excellent reflow after sputter method is revealed in a paper contributed by C. S. Park and five others to the proceedings Eighth VLSI Multilevel Interconnection Conference, 1991, pages 326 to 328, under the title of "Al-PLAPH (Aluminium-Planarization by Post-Heating) Process for Planarized Double Metal CMOS Applications".
In the manner which will later be described in greater detail, the process is a hole fill method of filling contact holes formed in an interlayer isolation layer laid on a substrate or a semiconductor device and having a principal surface and sidewalls defining the contact holes. The process comprises the steps of laying on the principal surface and on the sidewalls a covering film of a covering material consisting essentially of aluminium and of heating the covering film for reflow of aluminium of the covering material into the contact holes to spread a conductor film covering the principal surface and filling the contact holes. The covering film is laid by sputter deposition of the covering material in a sputter chamber. The covering film is heated in a reflow or annealing chamber. The substrate with the covering film on the isolation layer must therefore be transferred from the sputter chamber to the reflow chamber. Usually, a barrier metal film is preliminarily sputter deposited on the principal surface and on the sidewalls before sputter deposition of the covering film thereon.
In connection with the reflow after sputter method, it has been confirmed by the instant inventor that an aluminium oxide film is undesiredly formed along an outside surface of the covering film even if the substrate with the covering film is transferred from the sputter chamber into the reflow chamber through vacuum. The aluminium oxide film deteriorates reflow of aluminium of the covering material into the contact holes. It is possible to improve the reflow by heating the substrate to a high temperature. Use of the high temperature, however, gives rise to an objectionable chemical reaction through the barrier metal film between the covering film and a diffusion layer below the aluminium masses filling the contact holes. The chemical reaction, in turn, results in aluminium spikes to give rise to junction leakage currents. This adversely affects reliability and yield of the semiconductor device.